The present invention generally relates to an image reading apparatus having a line buffer, and more particularly to an image reading apparatus having a line buffer used in a facsimile set and so on.
An image reading apparatus, such as a scanner in a facsimile set, has a line image sensor in which a plurality of photoelectric conversion elements are arranged in a line. The line image sensor scans every line of a medium (such as a document) and outputs an image signal corresponding to an image formed on the medium.
An image reading apparatus has been proposed, in which the image signal output from the line image sensor is stored in a memory. The amount of information included in the image signal is very large, so that, in this type of image reading apparatus, the image signal is coded and compressed. Then, coded data which is obtained by the coding and the compressing of the image signal is stored in a memory.
Now, to increase the reading speed in the image reading apparatus, it is necessary at least either to increase the operation speed in the line image sensor, to increase the scanning speed of the line image sensor, or to increase the operation speed in an operation system in which the image signal is coded and compressed. The operation speed in the line image sensor is determined by the efficiency of a semiconductor device of the line image sensor. For example, in a case where the line image sensor scans the document while the document is being moved in a predetermined direction, the scanning speed is determined by the moving speed of the document. Thus, to increase the scanning speed, it is necessary to use a driving source such as a stepping motor in which a large driving force for moving the document can be generated. The operation speed in the operation system is determined by the efficiency of a microcomputer, a digital processing processor and so on which are used in the operation system.
Thus, in the conventional image reading apparatus, to increase the reading speed, at least either the operation speed in the line image sensor, the scanning speed of the line image sensor, or the operation speed in the operation system is improved with due regard to the total cost of the image reading apparatus.
In addition, another image reading apparatus has been proposed, in which the image signal output from the line image sensor is stored in a line buffer and then the image signal read out from the buffer is coded and compressed. According to this type of image reading apparatus, the process for coding and compressing the image signal can be performed by using a processor whose processing speed is relatively low.
However, the conventional image reading apparatus having the line buffer described above has the following disadvantage.
The time required for the coding and compressing of the image signal for one line changes in accordance with the amount of information represented by the image signal. Thus, in a case where the image signal for one line is stored in the buffer for a predetermined time, when the line image sensor successively scans the lines in which the required time for the coding and compressing of the image signal is large, there is a possibility that the line buffer will overflow with the image signal. In this case, before the line buffer overflows with the image signal, it is necessary to stop scanning the document. However, when the line image sensor starts scanning the document in the state where the scanning of the document is stopped, it is necessary to use a scanning system generating a large driving force for moving the document and it is also necessary for the line image sensor to stably scan the document from the start of the scanning. For example, when the scanning system has a stepping motor for moving the document, the starting torque of the stepping motor must be large. As a result, the cost of the scanning system increases.